1. Field of the Invention
The invention relates to a halogen incandescent capsule having a light-transmitting envelope which encloses a sealed cavity with a pinch at one end, and a filament having a pair of legs and a central barrel enclosed within the sealed cavity. The ends or legs of the filament are attached to a pair of lead wires which are sealed in the pinch. The lead wires extend out of the sealed cavity from the pinch. More particularly, the invention relates to a capsule having a filament with a primary coil and a secondary coil, where the primary coil ends form the legs for attachment to the leads.
2. Discussion of the Prior Art
FIG. 1 shows a known halogen incandescent capsule having a tungsten filament 10 with a barrel 16 between a pair of tungsten legs 18, and a xe2x80x9cdouble endedxe2x80x9d quartz envelope 20, with a pinch 22 at each end. The barrel 16 is located in a central cavity 24, and the coil legs 18 extend into the pinches 22 and are each welded to one end of a molybdenum foil 26. Molybdenum leads 28 are welded to the other end of the respective molybdenum foils 26 and extend out of the pinches 22.
To facilitate welding of the tungsten coiled legs 18 to the molybdenum foils 26, a small metal foil (platinum) 26A may be placed between the tungsten coil legs 18 and the molybdenum foil 26. The pinch 22 contains the molybdenum foil 26, the platinum foil 26A, as well as the ends of the respective tungsten coil legs 18 and molybdenum leads 28. The molybdenum foil is required in quartz envelopes 20 to create a gas-tight seal in the pinch 22 over the operating temperatures of the capsule.
FIG. 2 shows a conventional single-ended hard-glass capsule with a hard-glass envelope 30 and a pinch 32 at one end. Short and long molybdenum leads 36, 38 pass through the pinch 32. The short lead 36 is attached to one of the coil legs 18 typically with a clamp 37 formed in the molybdenum lead 36. The long lead 38 is attached to the coil leg 18 via clamp 39, for example. According to this known construction, both of the clamps 37, 39 as well as the entire tungsten filament 10 are located in the sealed cavity 34. The coefficient of thermal expansion of the hard-glass matches that of molybdenum eliminating the requirement of the molybdenum foils 26 shown in FIG. 1. Addition of molybdenum to the interior of the cavity 34 may require modification of the halogen chemistry to attenuate transport of molybdenum from the lead to the bulb wall.
FIG. 3 shows the filament 10 used in the halogen capsules shown in FIGS. 1 and 2. The coiled-coil filament 10 has a primary coil 14 and a secondary coil 16. The filament 10 is formed with a tungsten wire 12 wound on a primary mandrel having a diameter on the order of 80-150 xcexcm to form the primary coil 14 having an external diameter on the order of 100-300 xcexcm. The primary coil 14 is wound on a secondary mandrel having a diameter on the order of 300-800 xcexcm to form the secondary coil 16 which forms the barrel 16. The secondary mandrel is retracted or dissolved, and the primary mandrel is then removed in whole or in part by dissolving. U.S. Pat. No. 4,132,922 discloses a double-ended capsule having a so-called retained mandrel coil.
Double-ended quartz capsules are marketed in thin-glass outers, such as blown glass reflectors, decorative outers and the like for general lighting applications. Single-ended hard-glass capsules are marketed in thick-glass outers such as parabolic aluminized reflector (PAR) lamps and transmissive bulbs for general lighting. Double-ended quartz capsules with coil legs 18 extending into the press or pinch 22, as shown in FIG. 1, have passive extinction of electric arc at end-of-life. Elimination of non-passive failures in hard-glass halogen burners will enable marketing of thin-glass outer lamps containing the hard-glass burner.
It is an object of the invention to provide a halogen incandescent capsule in hard-glass which passively extinguishes arcing which occurs at end-of-life, with a simple and economic construction.
According to the invention, this and other objects are achieved by a halogen incandescent capsule with a hard-glass envelope having at least one pinch seal at one end thereof and containing a filament, e.g. a tungsten filament. At least one leg of the filament extends into the pinch seal and is attached to a lead, e.g., a molybdenum lead, in the pinch seal.
When the filament fails at end-of-life, the arc is extinguished passively with disintegration of the filament leg in the cavity and near the inside surface of the pinch seal.
According to a further aspect of the invention, the filament has a primary coil, where the primary coil of the filament leg is modified so that the leg portion in the pinch is straight or has an increased pitch. The modified, e.g., stretched, coil leg reduces the extinction time and electric arc energy at the end-of-life due to reduced linear wire density near the pinch.
Modifying, e.g., stretching out, the coil leg enables a robust clamping of the tungsten wire in the molybdenum clamp with complete closure of the clamp. This eliminates clamping on the primary winding which requires a tight tolerance gap within the molybdenum clamp, which in turn, eliminates strain in the clamped leg and fractures of the coil leg. Clamping on the modified coil leg negates the requirement of changeover time between wattages at the mount machine.
For this reason, it is advantageous to modify both coil legs where they are clamped, whether or not the clamps are located in a press seal.